1. Field of the Invention
The present invention relates to a control apparatus for a hydraulically operated vehicular transmission having a plurality of transmission trains to be established by a selecting operation of a plurality of hydraulic engaging elements. In this specification, the term "vehicular transmission" means a transmission for a vehicle such as a motor vehicle.
2. Description of the Related Art
As this kind of control apparatus, there has hitherto been known the following one. Namely, the hydraulic pressure of a hydraulic engaging element on disengaging side to be disengaged at the time of speed changing (disengaging pressure) and the hydraulic pressure of a hydraulic engaging element on engaging side to be engaged at the time of speed changing (engaging pressure) are electronically controlled by using solenoid proportional valves.
At the time of speed changing, especially at the time of upshifting, if the disengaging pressure is lowered before the engaging pressure has increased whereby the hydraulic engaging element on the engaging side has obtained a certain degree of engaging force, engine racing will occur to thereby cause speed change shocks.
As a solution, there is known a control apparatus in which a hydraulic pressure detecting means for detecting the engaging pressure is provided so that, when a detected value of the engaging pressure has exceeded a predetermined value to be set to such a value as will not cause engine racing, the disengaging pressure is reduced to a predetermined low pressure.
The above-described hydraulic pressure detecting means cannot be mounted on the hydraulic engaging element itself which is a rotational member. Therefore, it must be mounted on that upstream part of an oil passage which is connected to the hydraulic engaging element. As a result, between the detected value of the engaging pressure detected by the hydraulic pressure detecting means and the actual hydraulic pressure of the hydraulic engaging element on the engaging side, there will occur a difference corresponding to a pressure drop due to a flow resistance between the position at which the hydraulic pressure detecting means is mounted and the hydraulic engaging element. If the oil temperature is low, the viscosity of the oil increases and the pressure drop also increases. Therefore, at the time of low oil temperature, even if the detected value of the engaging pressure as detected by the hydraulic pressure detecting means has exceeded a predetermined value, the actual hydraulic pressure in the hydraulic engaging element on the engaging side has not sufficiently been increased. As a result, if the disengaging pressure is lowered at this point of time, the engine will race and speed change shocks will occur.
In view of the above point, the present invention has an object of providing a control apparatus in which the occurrence of speed change shocks at the time of low oil temperature can be prevented.